Polarization converter



Aug. 16, 1966 Filed Feb. 23. 1961 FIG. 1a

D. s. LERNER 3,26%,480

POLARI ZATION CONVERTER 5 Sheets-Sheet 1 lug 45 log f/fo Phase Shift 90lead FIG.3

Aug. 16, 1966 D. s. LERNER 3,267,430

POLARIZATION CONVERTER Filed Feb. 23. 1961 5 Sheets-Sheet 2 Aug. 16,1966 D. s. LERNER 3,

POLARIZATION CONVERTER Filed Feb. 23. 1961 5 Sheets-Sheet 3 FIG. 7a

51 Q99. or LP M. F F Tl I L as 63 i 67 1s 74 l t J7sl ill I I l I l FIG.7b FIG. 7c

United States Patent 3,267,480 POLARIZATION CONVERTER David S. Lerner,Smithtown, N.Y., assignor to Hazeltine Research, Inc., a corporation ofIllinois Filed Feb. 23, 1961, Ser. No. 91,133 8 Claims. (Cl. 343-911)This invention relates to transmission-type polarization converters andallows the conversion of any given polarization to any desiredpolarization. The invention will be described with particular referenceto a polarization converter adapted to allow an antenna designed tooperate with linear polarization to transmit and receive with circularpolarization.

Polarization conversion is important in many areas to which the presentinvention is applicable. For example, circular polarization is requiredfor various radar and communication applications; these include raindrop discrimination, propagation through the ionosphere (which mayrotate the polarization) and communication with a missile or satelliteof unknown or variable attitude. A transmission-type (as distinguishedfrom reflection-type) polarizer may be used to convert a linearlypolarized antenna to one with circular polarization and is applicable inthis regard to existing antennas as well as to antenna systems intendedfor circular polarization applications, but which can be designed moreconveniently and economically for linear polarization.

Many prior art antennas have utilized a lens in the form of anartificial dielectric. In applying this approach, an artificialdielectric is built up substituting discrete metal particles in place ofthe microscopic molecular particles existing in natural dielectrics. Bythe very nature of this approach a fairly substanital thickness of theartificial dielectric media is required to achieve useful results. Thisthickness is generally required to be several wave lengths at the designfrequency (in the absence of special techniques such as zoning which arenot applicable to the present problem). There have also been attempts toadapt this artificial dielectric idea to the design of polarizationconverters. One particular example suggested the use of twelve parallelsheets of conductive elements for operation as a quarter wave plate at afrequency in the vicinity of resonance of the elements. In thisarrangement all the elements had a principal dimension of substantiallyone-half of the operating wave length and the complete arrangement wasapproximately three operating wave lengths thick (i.e., in the directionof wave transmission).

The present invention utilizes a lumped element approach. The thicknessof a useful polarization converter constructed in accordance with thepresent invention can be made to depend only on the thickness requiredfor structural support. Thus, a quarter wave plate might, in practice,be a few thousandths of a wave length thick rather than several wavelengths thick as in the prior art. The type of polarization converterpresently considered to be the best mode of using the invention includesthree sheet arrays of elements to achieve extremely broad bandoperation. In one specific converter of this three sheet type, thethickness was approximately one-third of a wave length in the designfrequency range. While the comparison of a few thousandths of a wavelength in thickness to several wave lengths in thickness verydramatically points out the advantages of the present invention, thedifference between one-third a wave length and three or more wavelengths is still a very important advantage in the construction of apractical antenna system. It is obvious that great savings of space,weight, material, etc., result from the small size allowed.

The preceding discussion has completely ignored the 3,267,480 PatentedAugust 16, 1966 fact that prior art arrangements did not teach broadband impedance matching. Such matching is a very important additionaladvantage of the present invention.

It is an object of this invention, therefore, to provide new andimproved polarization converters.

It is a further object of this invention to provide polarizationconverters which avoid one or more disadvantages of the prior art andwhich are adapted to relatively wide frequency band-width operation.

It is an additional object of the invention to provide polarizationconverters which are physically simple, relatively inexpensive, and themajor components of which may be constructed using well known printedcircuit methods.

In accordance with the invention a polarization converter forelectromagnetic energy comprises a plurality of transmissivebirefringent repetitive sheet array of conductive elements includingelements which are discontinuous in all directions in combination withelements which are continuous in one direction, and means for supportingthe sheet arrays; the converter including sheet arrays of at least twodifferent configurations.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawing:

FIGS. la and 1b comprise two schematic representations useful indescribing the invention;

FIG. 2 is a graph illustrating certain characteristics of the circuitsof FIG. lb;

FIGS. 3 and 4 illustrate polarization converter sheets in accordancewith the present invention;

FIG. 5 is a schematic representation of a further embodiment of theinvention;

FIG. 6 is a graph illustrating certain characteristics of the circuitsof FIG. 5, and

FIGS. 7a, 7b and 7c comprise three views of a multiple sheetpolarization converter in accordance with the invention.

To facilitate explanation of the invention, the discussion will belimited for the moment to the special case of conversion of a linearlypolarized electromagnetic wave to a circularly polarized wave.

FIG. la illustrates arbitrary coordinates parallel to the wavefront of awave. A wave of any polarization may be analyzed in terms of componentsalong these coordinates. For example, a wave having a linearpolarization W, as shown in FIG. la, may be considered to comprise equalamplitude X and Y components. In this discussion, the X and Ycomponents, into which a wave may be broken down, will be referred to asthe components of this wave. The polarization of a wave may be convertedby shifting the phase of a particular component of the wave with respectto the remaining component. Thus referring to FIG. 1b, in such apolarization conversion we may regard the X and Y components of the waveas being transmitted through separate channels represented bytransmission lines as shown. 'In one line the input and output waveimpedanees are represented by resistances 10a and 10b, respectively, andin the other line by resistances 12a and 12b, respectively. In onetransmission line the input wave component (the Y component in thisexample) would be given a 45 phase lag by capacitance 11 of propermagnitude and supplied to resistor 10b. Simultaneously, the X componentof the input wave would be given a 45 phase lead by inductance 13 ofproper magnitude and supplied to resistor 12b. In this case, if the twooriginal X and Y components were components of a wave of polarization W,as shown in FIG. la, the signals appearing at resistors b and 12b couldbe recombined to produce a signal having circular polarization.

The results so produced are indicated in FIG. 2, wherein the verticalaxis appears at the frequency for which the circuits of FIG. lb weredesigned and at which these circuits produce leads and lags of exactly45. It will be seen that as the frequency of the electromagnetic wave isvaried, the phase shifts produced by capacitance 11 and inductance 13,represented by the Curves 20 and 21, respectively, in FIG. 2, change ina manner such that the relative phase shift produced in the two wavecomponents remains substantially equal to 90", as indicated by thedouble-headed arrow labeled 90 in FIG. 2. A properly designed system, asillustrated in FIG. lb, would be effective to convert a linearlypolarized wave at the design frequency and of proper aspect (i.e.,linearly polarized at 45 to the X and Y axes) to a circularly polarizedwave.

A circularly polarized wave may be represented as the sum of two equalamplitude linearly polarized waves sep arated in phase by 90. If eitheror both of these conditions i.e., equal amplitude and 90 phasedifference, are not present, the wave under consideration iselliptically polarized or in the special case of no phase difference,linearly polarized. If a wave of polarization W, as shown in FIG. la, isprocessed as described above by the circuits of FIG. lb, a circularlypolarized wave will result only at the design frequency. At any otherfrequency, the proper 90 phase shift will result, but these circuits donot have the same impedance magnitude except at the design frequency,and therefore, the components appearing at the resistances 10b and 1212will not be of equal amplitude and an elliptically polarized wave willbe produced at other frequencies. Thus, these particular circuitsexhibit wide frequency band width characteristics as far as phase shiftis concerned, but very narrow frequency band width characteristics asfar as amplitude equality is concerned.

Before referring to a specific embodiment of the invention, it will behelpful to have certain definitions in mind. For the purposes of thisspecification, transmissive indicates that polarization conversion takesplace as a result of transmission rather than reflection of a wave;birefringent is defined substantially as in optics, where it meanshaving differing indices of refraction along different axes; repetitive"refers to the characteristic of a repeating pattern; discontinuousrefers to a dimension of an element which is smaller than any operatingfrequency wave length; and continuous" refers to an element which has adimension in a plane perpendicular to the wave transmission axis whichis comparable to an over-all dimension of the complete polarizationconverter in this plane.

The fact that particular elements are physically continuous ordiscontinuous is of no real importance by itself. However, certainelectrical characteristics are inherent in elements with certainphysical characteristics and this physical manner of description isrelied upon because of the ease it allows in describing particularembodiments of the invention. This correlation between physical andelectrical characteristics will be further brought out in the followingdescription.

Referring now to FIG. 3, there is shown a polarization converter forelectromagnetic energy which comprises a single transmissivebirefringent repetitive sheet array of conductive elements includingelements 30, which are discontinuous in all directions, in combinationwith elements 31, which are continuous in one direction and means forsupporting these elements shown as a thin dielectric sheet 32. Thecomplete sheet, as illustrated, is in the form of a printed circuit, acopper layer having been etched so as to leave the rectangular patches30 and the narrow strips 31 on the surface of the thin supportingdielectric sheet 32. In operation, an electromagnetic wave passedthrough the polarization converter is acted upon much in the way itwould be if the X and Y components were actually separated and coupledto the circuits of FIG. lb. Thus, if the wavefront strikes the sheetwith a polarization W, as shown in FIG. 1a, the Y component (vertical)will be affected by shunt capacitances caused by the narrow verticalseparation 33 between adjacent rectangular elements 30. The Y componentwill be substantially unaffected by the parallel elements 31. The Xcomponent (horizontal) will be af fected by the shunt inductances causedby the parallel element 31. If the rectangular elements 30 have ahorizontal separation 34 between adjacent elements which is large, thecapacitive effect of the elements 30 will be negligible with respect tothe X component. Thus, the sheet shown in FIG. 3 accomplishes the resultof the circuits of FIG. 1b as shown in FIG. 2. As discussed withreference to FIG. 1]), this arrangement has wide frequency band widthcharacteristics with regard to phase shift but rather poor impedancematching characteristics, with the result that energy of proper linearpolarization and frequency transmitted through this sheet will receivethe desired polarization transformation. However, the efficiency of thistransmission will be rather poor and any input wave of 45 linearpolarization but incorrect frequency will result in an ellipticallypolarized output wave.

All sheet arrays of conductive elements, as illustrated in FIG. 3, willbe resonant at some particular frequency. However, the operatingfrequency range of such polarization converters, in accordance with thisinvention, is substantially below the resonant frequency so as not torely on any of the effects directly resulting from resonance.Substantially below" is intended to indicate that converters do not relyupon resonant effects as did certain prior art arrangements. Practicalarrangements will, in fact, generally have an operating frequency rangewhich lies below three-quarters of the resonant frequency of the sheetarrays. That is to say, for example, if a particular sheet array isdesigned with an operating range in the vicinity of 2500 megacycles persecond, any resonance of this sheet array will occur above approximately3300 megacycles per second in most designs in accordance with theinvention.

With reference now to FIG. 4, there is illustrated a transmissivebirefringent repetitive sheet array in which conductive elements whichare discontinuous in all dimensions (i.e. elliptical patches whichcorrespond to the rectangular patches 30 of FIG. 3) have been physicallycombined with conductive elements which are continuous in one dimension(i.e. thin strips corresponding to the thin strips 31 of FIG. 3) to formthe illustrated pattern of elements 38. This arrangement may be designedto perform in the same manner as the FIG. 3 arrangement and is includedto illustrate two points. First, that elements which are discontinuousin all directions should not be considered as limited merely to therectangular patches 30 of FIG. 3 but may, in fact, be of any desiredshape. And second, that the term in combination with refers to acombination of two distinct unattached types of elements, as in FIG. 3,as well as to physical combinations of the distinct types of elementsinto a single element, such as the elements 38 in FIG. 4.

In accordance with the invention it is possible to constructpolarization converters utilizing a plurality of sheet arrays ofconductive elements. One way of doing this would be to provide one typeof sheet with only inductive elements and a second type of sheet bearingonly capacitive elements. Using such sheets, it is possible to constructa polarization converter which is symmetrical along the axis ofpropagation and which includes four sheetstwo with capacitive elementsand two with inductive elements. This separation of the inductiveelements from the capacitive elements allows the inductances andcapacitances to be placed at proper separations along the propagationpath, since in a converter using two inductive sheet arrays and twocapacitive sheet arrays the optimum separation of the capacitive sheetswill be less than a quarter wave length and the optimum separation ofthe inductive sheets will be greater than a quarter wave length. Thisarrangement will allow a relatively wide band polarization conversionwith a somewhat reduced reflection coefficient over this band width butwith the disadvantage of requiring four separate sheet arrays.

It is desired that a number of criteria be optimized in the design of apolarization converter, including: accurate polarization conversion withrespect to phase over a wide frequency band width; accurate impedancematching over a wide frequency band width; symmetry along the directionof energy propagation; and the economical requirement of the smallestpossible number of individual sheet arrays. Referring now to FIG. 5there is shown schematically an arrangement which effectively meetsthese criteria. In this arrangement the Y components are arranged to becoupled to the tandem combination of three capacitances and the Xcomponents are arranged to be coupled to three individual shuntcircuits, each comprising the parallel combination of an inductance anda capacitance. Symmetry is desired to simplify design computation and toprovide economy by reducing the number of different types of sheetarrays required in a converter. To obtain symmetry along the directionof propagation, it is necessary that capacitance 40:: be identical tocapacitance 40b; and that capacitance 42a and inductance 43a beidentical to capacitance 42b and inductance 43b, respectively. Thus, inthe design of the circuits of FIG. 5 seven variables must be accountedfor; the magnitudes of each of capaoitances 40, 41, 42 and 44; themagnitudes of each of inductances 43 and 45; and the separation 46between the components. For a symmetrical structure, seven variables aremore than actually required, four being required to match for twopolarizations at each of two frequencies in the desired pass band andtwo being required to provide 90 phase shift at the two frequencies,thus leaving one variable for optimization of relative frequencyresponse over the pass band at the two polarizations. The design processis rather complicated but can be carried out using known techniques bythose skilled in the art once the present concepts are understood.

Referring to FIG. 6, there are shown curves representative of theoperation of a polarization converter effectively similar to thecircuits of FIG. 5. This design is such that the effect of the threeshunt capacitauces is to produce 90 phase lag in the Y components of aninput wave of predetermined frequency as shown by Curve 50 of FIG. 6;and the three shunt arranged parallel inductance-capacitancecombinations produce zero phase shaft in the X components as shown byCurve 51. When the frequency of the input wave varies from thepredetermined frequency, the characteristics of both circuits of FIG. 4change in essentially identical manner and the relative phase shift canbe caused to remain substantially constant at a desired value-90 in thiscase, as indicated by the double headed arrow labeled 90.

With reference now to FIG. 7, there are shown three views of an actualpolarization converter constructed in accordance with the invention, andhaving characteristics substantially as indicated in FIG. 6. FIG. 7a isa side view of a complete polarization converter including a pluralityof transmissive birefringent repetitive sheet arrays of conductiveelements separated by a dielectric medium. Thus, the converter of FIG.7a includes a sheet 60a, an identical sheet 60b, a third sheet 61 and aseparating dielectric in the form of a thin walled insulating materialin the configuration of a honeycomb including relatively large volumesof air. This is shown by inset 62, which is an end view of a section ofthe honeycomb material shown in side view in the main part in FIG. 7a.As shown in FIG. 7b, sheet 61 includes conductive elements substantiallysimilar to those utilized in the FIG. 3 arrangement, the principaldifference being that there are relatively small separations betweenadjacent elements 63 thereby providing sizable capacitive effects forboth X and Y wave components. One of the sheets 60 is illustrated inFIG. 70. The configuration of this sheet is very similar to that ofsheet 61 except that the different capacitive and inductive eflectsdesired require elements, which are discontinuous in all directions, ofa different shape. Referring again to FIG. 7a, it will be appreciatedthat the separation 65 between individual sheets corre sponds toseparation 46 in FIG. 5.

The views of FIG. 7 are essentially scale drawings of a polarizationconverter which was actually constructed, except that certain dimensionshave been distorted in the interest of clarity in the drawing. Theactual dimensions in inches were as follows (the patterns aresymmetrical and these values are typical):

Each of the sheets 60a, 60b and 61 took the form of a sheet of glassfiber laminate .012 inch thick with copper elements .003 inch thickformed thereon by printed circuit etching methods. The honeycomb-airseparating dielectric had a dielectric constant of approximately 1.06 Aform or expanded insulative material can be substituted for thishoneycomb material and in some instances it may be desirable to have theconductive elements supported directly by this low dielectric constantmeans, without the thin sheets of higher dielectric constant used inthis example. This polarization converter was effective to bilaterallyconvert a linearly polarized incident wave of proper aspect to acircularly polarized wave, or a circularly polarized incident wave to alinearly polarized wave. The converter was effective over a band from2420 megacycles to 2930 megacycles with theoretical values of relativephase shift between X and Y of i1 and reflection coefiicient of lessthan .025 over this band. Actual measurements tended to confirm thesetheoretical values but were limited by the accuracy of availablemeasuring techniques and equipment.

With the basic ideas of the invention now in mind, a number of factswhich are self-evident, nevertheless seem worthy of mention. curved inforn i gs reqnir pfr 'ipaff'tidfilaf'applicatiph'sf i'cTrE'irarnple, acompleteconve'rter' niay used are protective covering for an antenna orhorn much in the manner of well-known radomes. It will m appreciatedthat the broad-band characteristics, of converters in accordance withthis invention, allow efiicient operation even though the propagationdirection of an incident wave is not strictly perpendicular to theindividual sheet or sheets of a converter. Also it may be desirable toconstruct converters in accordance with the invention which use aplurality of arrays of either elements which are discontinuous in alldirections or elements which are continuous in one direction, withoutincluding any elements of the other type. In these cases the elements ofeach sheet array may be aligned in one common direction so that eacharray produces its principal effect in a common direction.

Although the invention has been described with particular reference to aconverter for changing linearly polarized energy to circularly polarizedenergy and vice versa, it should be appreciated that this is merely aspecific application and that the invention may actually be employed toconvert energy of any given polarization All arrangements may be fiatorto any other desired polarization. It is, therefore, intended, in theappended claims, to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:

1. A polarization converter for electromagnetic energy comprising: aplurality of transmissive birefringent repetitive sheet arrays ofconductive elements, including elements which are discontinuous in alldirections in combination with elements which are continuous in onedirection; and means for supporting said sheet arrays; said converterincluding sheet arrays of at least two different configurations.

2. A polarization converter for electromagnetic energy comprising: aplurality of transmissive birefringent repetitive sheet arrays ofconductive elements, each sheet array including elements which arediscontinuous in all directions and elements which are continuous in onedirection; and means for supporting said sheet arrays; said converterincluding sheet arrays of at least two different configurations.

3. A polarization converter for electromagnetic energy comprising. aplurality of transmissive birefringent repetitive sheet arrays ofconductive elements, each sheet array including elements which arediscontinuous in all directions in combination with elements which arecontinuous in one direction; and means for supporting said sheet arrays;the converter having an operating frequency range which is below theresonant frequency of all of said sheet arrays; said converter includingsheet arrays of at least two different configurations.

4. A polarization converter for electromagnetic energy comprising: aplurality of sheets of insulative material; a birefringent repetitivearray of conductive elements, including elements which are discontinuousin all directions in combination with elements which are continuous inone direction, adhered to each of said sheets; and means for supportingsaid sheets; said converter including sheet arrays of at least twodifferent configurations.

5. A polarization converter for electromagnetic energy comprising: aplurality of transmissive birefringent repetitive sheet arrays ofconductive elements, including elements which are discontinuous in alldirections in combination with elements which are continuous in onedirection; and low dielectric constant means for supporting said sheetarrays; said converter including sheet arrays of at least two differentconfigurations.

6. A polarization converter for electromagnetic energy comprising: aplurality of transmissive birefringent repetitive sheet arrays ofconductive elements, including elements which are discontinuous in alldirections in combination with elements which are continuous in onedirection; and means for supporting said sheet arrays and for forming aprotective covering for an antenna; said converter including sheetarrays of at least two different configurations.

7. A quarterwave plate for electromagnetic energy comprising: threesheets of insulative material; a transmissive birefringent repetitivesheet array of conductive elements including elements which arediscontinuous in all directions in combination with elements which arecontinuous in one direction, adhered to each of said sheets; and lowdielectric constant means for supporting said sheets; said converterincluding sheet arrays of at least two different configurations.

8. A polarization converter for electromagnetic energy comprising: aplurality of transmissive birefringent repetitive arrays of thinsubstantially two-dimensional conductive elements arranged in aplurality of sheets, including first conductive elements all of whosedimensions are smaller than the operating wavelength in combination withsecond conductive elements having a single dimension in the planeperpendicular to the wave transmission axis which is comparable to anover-all dimension of the complete polarization converter in said plane;and means for supporting said elements; said converter including arraysof at least two different configurations.

References Cited by the Examiner UNITED STATES PATENTS 2,756,424 7/1956Lewis et al 343-909 2,921,312 1/1960 Wickersham 343-911 2,978,702 4/1961Pakan 343909 X 3,089,142 5/1963 Wickersham 3439l1 3,092,834 6/1963McCann et al. 343-756 HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, E. LIEBERMAN,

Assistant Examiners.

1. A POLARIZATION CONVERTER FOR ELECTROMAGNETIC ENERGY COMPRISING: APLURALITY OF TRANSMISSIVE BIREFRINGENT REPETITIVE SHEET ARRAYS OFCONDUCTIVE ELEMENTS, INCLUDING ELEMENTS WHICH ARE DISCONTINUOUS IN ALLDIRECTIONS IN COMBINATION WITH ELEMENTS WHICH ARE CONTINUOUS IN ONEDIRECTION; AND MEANS FOR SUPPORTING SAID SHEET ARRAYS; SAID CONVERTERINCLUDING SHEET ARRAYS OF AT LEAST TWO DIFFERENT CONFIGURATIONS.